[llvm] efdd089 - [LoopInterchange] Reject if outer loop reduction has extra user (#203241)
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Thu Jun 11 08:47:32 PDT 2026
Author: Ryotaro Kasuga
Date: 2026-06-11T15:47:27Z
New Revision: efdd089088747136ebf134f4fc0b56ccf25bb118
URL: https://github.com/llvm/llvm-project/commit/efdd089088747136ebf134f4fc0b56ccf25bb118
DIFF: https://github.com/llvm/llvm-project/commit/efdd089088747136ebf134f4fc0b56ccf25bb118.diff
LOG: [LoopInterchange] Reject if outer loop reduction has extra user (#203241)
As demonstrated by the case reported in #203104, there are cases where
interchanging the loops is unsafe because the outer loop reduction is
used within the inner loop by something other than the PHI node that
receives the value. Such cases should be rejected during the legality
check phase.
This patch adds a check to ensure that the outer loop reduction is only
used by the inner loop reduction PHI node. This check might be too
conservative and could cause missed optimizations, but at the moment I
can't find such a case.
Fixes #203104.
Added:
Modified:
llvm/lib/Transforms/Scalar/LoopInterchange.cpp
llvm/test/Transforms/LoopInterchange/reduction-extra-use-in-inner-loop.ll
Removed:
################################################################################
diff --git a/llvm/lib/Transforms/Scalar/LoopInterchange.cpp b/llvm/lib/Transforms/Scalar/LoopInterchange.cpp
index 5d44032b47195..3da4e99860e45 100644
--- a/llvm/lib/Transforms/Scalar/LoopInterchange.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopInterchange.cpp
@@ -1270,8 +1270,26 @@ bool LoopInterchangeLegality::checkInductionsAndReductions(Loop *OuterLoop) {
PHI.getIncomingValueForBlock(OuterLoop->getLoopLatch()));
PHINode *InnerRedPhi = findInnerReductionPhi(
InnerLoop, V, HasNoWrapReductions, HasNoInfInsts);
+
+ // Reject if PHI has users other than InnerRedPhi. The typical case is
+ // as follows:
+ //
+ // o.header:
+ // %red.o = phi [ 0, ... ], [ %red.next, %o.latch ]
+ // br label %i.header
+ //
+ // i.header:
+ // %red.i = phi [ %red.o, %o.header ], [ %red.next, %i.latch ]
+ // br label %i.body
+ //
+ // i.body:
+ // store %red.o to %mem
+ // ...
+ //
if (!InnerRedPhi ||
- !llvm::is_contained(InnerRedPhi->incoming_values(), &PHI)) {
+ !llvm::is_contained(InnerRedPhi->incoming_values(), &PHI) ||
+ !all_of(PHI.users(),
+ [InnerRedPhi](User *U) { return U == InnerRedPhi; })) {
LLVM_DEBUG(
dbgs()
<< "Failed to recognize PHI as an induction or reduction.\n");
@@ -1284,6 +1302,7 @@ bool LoopInterchangeLegality::checkInductionsAndReductions(Loop *OuterLoop) {
});
return false;
}
+
OuterInnerReductions.insert(&PHI);
OuterInnerReductions.insert(InnerRedPhi);
} else {
diff --git a/llvm/test/Transforms/LoopInterchange/reduction-extra-use-in-inner-loop.ll b/llvm/test/Transforms/LoopInterchange/reduction-extra-use-in-inner-loop.ll
index ecdf95f32cb2c..82ba8e234c5da 100644
--- a/llvm/test/Transforms/LoopInterchange/reduction-extra-use-in-inner-loop.ll
+++ b/llvm/test/Transforms/LoopInterchange/reduction-extra-use-in-inner-loop.ll
@@ -23,45 +23,33 @@
;
; In this case, `sum_i` evolves 0 -> 1, while `sum_j` evolves 0 -> 0.
;
-; FIXME: The loops can be interchanged now.
-;
define i8 @extra_reduction_use_in_inner0(ptr noalias %A, ptr noalias %B) {
; CHECK-LABEL: define i8 @extra_reduction_use_in_inner0(
; CHECK-SAME: ptr noalias [[A:%.*]], ptr noalias [[B:%.*]]) {
-; CHECK-NEXT: [[INNER_PREHEADER:.*:]]
+; CHECK-NEXT: [[INNER_PREHEADER:.*]]:
; CHECK-NEXT: br label %[[INNER:.*]]
-; CHECK: [[OUTER_HEADER_PREHEADER1:.*]]:
+; CHECK: [[INNER]]:
+; CHECK-NEXT: [[I:%.*]] = phi i64 [ 0, %[[INNER_PREHEADER]] ], [ [[I_NEXT:%.*]], %[[OUTER_LATCH:.*]] ]
+; CHECK-NEXT: [[SUM_OUTER:%.*]] = phi i8 [ 0, %[[INNER_PREHEADER]] ], [ [[SUM_LCSSA:%.*]], %[[OUTER_LATCH]] ]
; CHECK-NEXT: br label %[[OUTER_HEADER_PREHEADER:.*]]
; CHECK: [[OUTER_HEADER_PREHEADER]]:
-; CHECK-NEXT: [[I:%.*]] = phi i64 [ [[I_NEXT:%.*]], %[[OUTER_LATCH:.*]] ], [ 0, %[[OUTER_HEADER_PREHEADER1]] ]
-; CHECK-NEXT: [[SUM_INNER:%.*]] = phi i8 [ [[SUM_NEXT:%.*]], %[[OUTER_LATCH]] ], [ [[SUM:%.*]], %[[OUTER_HEADER_PREHEADER1]] ]
-; CHECK-NEXT: br label %[[INNER_SPLIT1:.*]]
-; CHECK: [[INNER]]:
-; CHECK-NEXT: br label %[[INNER1:.*]]
-; CHECK: [[INNER1]]:
-; CHECK-NEXT: [[J:%.*]] = phi i64 [ [[TMP2:%.*]], %[[INNER_SPLIT:.*]] ], [ 0, %[[INNER]] ]
-; CHECK-NEXT: [[SUM]] = phi i8 [ [[SUM_LCSSA:%.*]], %[[INNER_SPLIT]] ], [ 0, %[[INNER]] ]
-; CHECK-NEXT: br label %[[OUTER_HEADER_PREHEADER1]]
-; CHECK: [[INNER_SPLIT1]]:
+; CHECK-NEXT: [[J:%.*]] = phi i64 [ 0, %[[INNER]] ], [ [[TMP0:%.*]], %[[OUTER_HEADER_PREHEADER]] ]
+; CHECK-NEXT: [[SUM_INNER:%.*]] = phi i8 [ [[SUM_OUTER]], %[[INNER]] ], [ [[SUM_INNER_NEXT:%.*]], %[[OUTER_HEADER_PREHEADER]] ]
; CHECK-NEXT: [[GEP_A:%.*]] = getelementptr [2 x i8], ptr [[A]], i64 [[J]], i64 [[I]]
; CHECK-NEXT: [[A:%.*]] = load i8, ptr [[GEP_A]], align 1
-; CHECK-NEXT: [[SUM_NEXT]] = add i8 [[SUM_INNER]], [[A]]
+; CHECK-NEXT: [[SUM_INNER_NEXT]] = add i8 [[SUM_INNER]], [[A]]
; CHECK-NEXT: [[GEP_B:%.*]] = getelementptr [2 x i8], ptr [[B]], i64 [[J]], i64 [[I]]
-; CHECK-NEXT: store i8 [[SUM]], ptr [[GEP_B]], align 1
-; CHECK-NEXT: [[TMP0:%.*]] = add i64 [[J]], 1
+; CHECK-NEXT: store i8 [[SUM_OUTER]], ptr [[GEP_B]], align 1
+; CHECK-NEXT: [[TMP0]] = add i64 [[J]], 1
; CHECK-NEXT: [[TMP1:%.*]] = icmp eq i64 [[TMP0]], 2
-; CHECK-NEXT: br label %[[OUTER_LATCH]]
-; CHECK: [[INNER_SPLIT]]:
-; CHECK-NEXT: [[SUM_LCSSA]] = phi i8 [ [[SUM_NEXT]], %[[OUTER_LATCH]] ]
-; CHECK-NEXT: [[TMP2]] = add i64 [[J]], 1
-; CHECK-NEXT: [[TMP3:%.*]] = icmp eq i64 [[TMP2]], 2
-; CHECK-NEXT: br i1 [[TMP3]], label %[[EXIT:.*]], label %[[INNER1]]
+; CHECK-NEXT: br i1 [[TMP1]], label %[[OUTER_LATCH]], label %[[OUTER_HEADER_PREHEADER]]
; CHECK: [[OUTER_LATCH]]:
+; CHECK-NEXT: [[SUM_LCSSA]] = phi i8 [ [[SUM_INNER_NEXT]], %[[OUTER_HEADER_PREHEADER]] ]
; CHECK-NEXT: [[I_NEXT]] = add i64 [[I]], 1
; CHECK-NEXT: [[EC_I:%.*]] = icmp eq i64 [[I_NEXT]], 2
-; CHECK-NEXT: br i1 [[EC_I]], label %[[INNER_SPLIT]], label %[[OUTER_HEADER_PREHEADER]]
+; CHECK-NEXT: br i1 [[EC_I]], label %[[EXIT:.*]], label %[[INNER]]
; CHECK: [[EXIT]]:
-; CHECK-NEXT: [[RES:%.*]] = phi i8 [ [[SUM_LCSSA]], %[[INNER_SPLIT]] ]
+; CHECK-NEXT: [[RES:%.*]] = phi i8 [ [[SUM_LCSSA]], %[[OUTER_LATCH]] ]
; CHECK-NEXT: ret i8 [[RES]]
;
entry:
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